290 THE EVOLUTION THEORY 



l)ut iK'\ertholoss the spindle threads penetrate into the interior of 

 tlu' niicleus. But the chromosomes always range themselves quite 

 re*mlarlv in the ' eqiiatorial plane' of the spindle {D, aeq) — a process 

 the precise mechanism of which is by no means clearly understood, 

 and indeed the play of the forces in the whole process of nuclear 

 division is still very imperfectly revealed to our intelligence. 



Thus we have now before us a pale, spindle-shaped figure, which 

 takes only a faint stain, with the ' suns ' ((\s) at its ' poles,' and in its 

 equatorial plane the loop- or rod-shaped, or spherical chromosomes 

 (c7(!)>). The whole is designated the ' karyokinetic,' the 'mitotic,' or 

 the ' nuclear division figure.' 



The meaning and importance of this, at first sight, puzzling figure 

 will at once become clear from what follows. It may be observed 

 at this stage, if not even long before, that each of the chromatin rods 

 or loops has split along its whole length like a log of wood, and that 

 the split halves are beginning slowly and hardly noticeably to move 

 away from each other, one half towards one, the other towards the 

 other pole of the spindle (Fig. D and F). Directly in front of the 

 centrosome they make a halt, and now the material for the two 

 daughter - nuclei is in its proper place [F, cltrs). These develop 

 quickly, each chromosome group surrounding itself with a nuclear 

 membrane (Fig. G) within which the chromosomes gradually become 

 transformed again into a nuclear network. Within the chromatin 

 substance proper this is scattered about in small roundish or angular 

 granules, lying especially at the intersecting points of the network. 

 It may be stated at once, though the full significance of the statement 

 can only be appreciated later, that we may assume with probability 

 that this breaking up of the chromosomes is only apparent, and that 

 these rods or spheres really continue to exist in the nuclear network, 

 only in a difterent form, greatly spread out, somewhat after the 

 manner of a Rhizopod which stretches out fine processes in all 

 directions. These processes branch and anastomose, so that the body, 

 which previously seemed compact, now appears as a fine network. 

 In point of fact, it can be directly observed that the chromosomes, 

 after the nucleus is completely divided into two daughter-nuclei, send 

 out pointed processes {F and G) which gradually increase in length and 

 ])ranch, while the body of the chromosome itself becomes gradually- 

 smaller. It is thus probable that, when such a daughter-nucleus 

 is on the point of dividing anew, it may, by a drawing together of the 

 processes or pseudopodia of the chromosomes, produce the same rods 

 or spheres as those which previously gave rise to the network. More 

 definite reasons for this interpretation will be adduced later on. In 



